Electrostatic traps are a class of ion optical devices where moving ions experience multiple reflections or deflections in substantially electrostatic fields. Unlike for trapping in RF field ion traps, trapping in electrostatic traps is possible only for moving ions. Thus, a high vacuum is required to ensure that this movement takes place with minimal loss of ion energy due to collisions over a data acquisition time Tm. Since its commercial introduction in 2005, the ORBITRAP™ mass analyzer, which belongs to the class of electrostatic trap mass analyzers, has become widely recognized as a useful tool for mass spectrometric analysis. In brief, the ORBITRAP™ mass analyzer, which is commercially available from Thermo Fisher Scientific of Waltham Mass. USA, is a type of electrostatic trap mass analyzer that is substantially modified from the earlier Kingdon ion trap. FIGS. 1A and 1B, discussed further below, provide schematic illustrations of an ORBITRAP™ mass analyzer. The main advantages of electrostatic trapping mass analyzers of the type illustrated in FIGS. 1A-1B and of mass spectrometer systems that incorporate such mass analyzers are that they provide accurate mass-to-charge (m/z) measurements and high m/z resolution similar to what is achievable with Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry instrumentation but without the requirement for a high-strength magnet. Structural and operational details of ORBITRAP™ mass analyzers and mass spectrometers employing such mass analyzers are described in Makarov, Electrostatic Axially Harmonic Orbital Trapping: A High-Performance Technique of Mass Analysis, Anal. Chem., 72(6), 2000, pp. 1156-1162 and in U.S. Pat. No. 5,886,346 in the name of inventor Makarov and in U.S. Pat. No. 6,872,938 in the names of inventors Makarov et al.
In both FT-ICR and ORBITRAP™ mass analyzers, ions are compelled to undergo collective oscillatory motion within the analyzer which induces a correspondingly oscillatory image charge in neighboring detection electrodes, thereby enabling detection of the ions. The oscillatory motion used for detection may be of various forms including, for example, circular oscillatory motion in the case of FT-ICR and axial oscillatory motion while orbiting about a central electrode in the case of a mass analyzer of the type schematically illustrated in FIGS. 1A-1B or a mass spectrometer employing such a mass analyzer. The oscillatory image charge in turn induces an oscillatory image current and corresponding voltage in circuitry connected to the detection electrodes, which is then typically amplified, digitized and stored in computer memory which is referred to as a transient (i.e. a transitory signal in the time domain). The oscillating ions induce oscillatory image charge and oscillatory current at frequencies which are related to the mass-to-charge (m/z) values of the ions. Each ion of a given mass to charge (m/z) value will oscillate at a corresponding given frequency such that it contributes a signal to the collective ion image current which is generally in the form of a periodic wave at the given frequency. The total detected image current of the transient is then the resultant sum of the image currents at all the frequencies present (i.e. a sum of periodic signals). Frequency spectrum analysis (such as Fourier transformation) of the transient yields the oscillation frequencies associated with the particular detected oscillating ions; from the frequencies, the m/z values of the ions can be determined (i.e. the mass spectrum) by known equations with parameters determined by prior calibration experiments.
More specifically, an ORBITRAP™ mass analyzer includes an outer barrel-like electrode and a central spindle-like electrode along the axis. Referring to FIG. 1A, a portion of a mass spectrometer system including an ORBITRAP™ mass analyzer is schematically shown in longitudinal section view. The mass spectrometer system 1 includes an ion injection device 2 and an electrostatic orbital trapping mass analyzer 4. The ion injection device 2, in this case, is a curved multipolar curvi-linear trap (known as a “C-trap”). Ions are ejected radially from the “C-trap” in a pulse to the Orbitrap. For details of the curved trap, or C-trap, apparatus and its coupling to an electrostatic trap, please see U.S. Pat. Nos. 6,872,938; 7,498,571; 7,714,283; 7,728,288; and 8,017,909 each of which is hereby incorporated herein by reference in its entirety. The C-trap may receive and trap ions from an ion source 3 which may be any known type of source such as an electrospray (ESI) ion source, a Matrix-Assisted Laser Desorption Ionization (MALDI) ion source, a Chemical Ionization (CI) ion source, an Electron Ionization (EI) ion source, etc. Additional not-illustrated ion processing components such as ion guiding components, mass filtering components, linear ion trapping components, ion fragmentation components, etc. may optionally be included (and frequently are included) between the ion source 3 and the C-trap 2 or between the C-trap and other parts of the mass spectrometer. Other parts of the mass spectrometer which are not shown are conventional, such as additional ion optics, vacuum pumping system, power supplies etc.
Other types of ion injection devices may be employed in place of the C-trap. For example, the aforementioned U.S. Pat. No. 6,872,938 teaches the use of an injection assembly comprising a segmented quadrupole linear ion trap having an entrance segment, an exit segment, an entrance lens adjacent to the entrance segment and an exit lens adjacent to the exit segment. By appropriate application of “direct-current” (DC) voltages on the two lenses as well as on the rods of each segment, a temporary axial potential well may be created in the axial direction within the exit segment. The pressure inside the trap is chosen in such a way that ions lose sufficient kinetic energy during their first pass through the trap such that they accumulate near the bottom of the axial potential well. Subsequent application of an appropriate voltage pulse to the exit lens combined with ramping of the voltage on a central spindle electrode causes the ions to be emptied from the trap axially through the exit lens electrode and to pass into the electrostatic orbital trapping mass analyzer 4.
The electrostatic orbital trapping mass analyzer 4 comprises a central spindle shaped electrode 6 and a surrounding outer electrode which is separated into two halves 8a and 8b. FIG. 1B is an enlarged cross-sectional view of the inner and outer electrodes. The annular space 17 between the inner spindle electrode 6 and the outer electrode halves 8a and 8b is the volume in which the ions orbit and oscillate and comprises a measurement chamber in that the motion of ions within this volume induces the measured signal that is used to determine the ions m/z ratios and relative abundances. The internal and external electrodes (electrodes 6 and 8a, 8b) are specifically shaped such that, when supplied with appropriate voltages will produce respective electric fields which interact so as to generate, within the measurement chamber 17, a so-called “quadro-logarithmic potential”, U, (also sometimes referred to as a “hyper-logarithmic potential”) which is described in cylindrical coordinates (r, z) by the following equation:
                    U        =                                            a              2                        ⁢                          (                                                z                  2                                -                                                      r                    2                                    2                                            )                                +                      b            ⁢                                                  ⁢                          ln              ⁡                              (                                  r                  c                                )                                              +          d                                    Eq        .                                  ⁢        1            where a, b, c, and d are constants determined by the dimensions of and the voltage applied to the orbital trapping analyzer electrodes, where z=0 is taken at the axial position corresponding to the equatorial plane of symmetry 7 of the electrode structure and chamber 17 as shown in FIG. 1B. The “bottom” or zero axial gradient point of the portion of “quadro-logarithmic potential” dependent on the axial displacement (i.e. the portion which determines motion in the axial dimension, z, along the longitudinal axis 9) occurs at the equatorial plane 7. This potential field has a harmonic potential well along the axial (Z) direction which allows an ion to be trapped axially within the potential well if it does not have enough kinetic energy to escape. It should be noted that Eq. 1 represents an ideal functional form of the electrical potential and that the actual potential in any particular physical apparatus will include higher-order terms in both z and r.
The motions of trapped ions are associated with three characteristic oscillation frequencies: a frequency of rotation around the central electrode 6, a frequency of radial oscillations a nominal rotational radius and a frequency of axial oscillations along the z-axis. In order to detect the frequencies of oscillations, the motion of ions of a given m/z need to be coherent. The radial and rotational oscillations are only partially coherent for ions of the same m/z as differences in average orbital radius and size of radial oscillations correspond to different orbital and radial frequencies. It is easiest to induce coherence in the axial oscillations as ions move in an axial harmonic potential so axial oscillation frequency is independent of oscillation amplitude and depends only on m/z and, therefore, the axial oscillation frequencies are the only ones used for mass-to-charge ratio determinations. The outer electrode is formed in two parts 8a, 8b as described above and is shown in FIG. 1B. The ions oscillate sinusoidally with a frequency, ω, (harmonic motion) in the potential well of the field in the axial direction according to the following Eq. 2:
                    ω        =                              k                          (                              m                /                z                            )                                                          Eq        .                                  ⁢        2            where k is a constant. One or both parts 8a, 8b of the outer electrode are used to detect image current as the ions oscillate back and forth axially. The Fourier transform of the induced ion image current signal from the time domain to the frequency domain can thus produce a mass spectrum in a conventional manner. This mode of detection makes possible high mass resolving powers.
Ions having various m/z values which are trapped within the C-trap are injected from the C-trap into the electrostatic orbital trapping mass analyzer 4 in a temporally and spatially short packet at an offset ion inlet aperture 5 that is located at an axial position which is offset from the equatorial plane 7 of the analyzer in order to achieve “excitation by injection” whereby the ions of the ion packet immediately commence oscillation within the mass analyzer in the quadro-logarithmic potential. The ions oscillate axially between the two outer electrodes 8a and 8b while also orbiting around the inner electrode 6. The axial oscillation frequency of an ion is dependent on the m/z values of the ions contained within the ion packet so that ions in the packet with different m/z begin to oscillate at different frequencies.
The two outer electrodes 8a and 8b serve as detection electrodes. The oscillation of the ions in the mass analyzer causes an image charge to be induced in the electrodes 8a and 8b and the resulting image current in the connected circuitry is picked-up as a signal and amplified by an amplifier 10 (FIG. 1A) connected to the two outer electrodes 8a and 8b which is then digitized by a digitizer 12. The resulting digitized signal (i.e. the transient) is then received by an information processor 14 and stored in memory. The memory may be part of the information processor 14 or separate, preferably part of the information processor 14. For example, the information processor 14 may comprise a computer running a program having elements of program code designed for processing the transient. The computer 14 may be connected to an output means 16, which can comprise one or more of: an output visual display unit, a printer, a data writer or the like.
The transient received by the information processor 14 represents the mixture of the image currents produced by the ions of different m/z values which oscillate at different frequencies in the mass analyzer. A transient signal for ions of one m/z is periodic as shown in FIG. 2A, which shows a “symbolic” approximately sinusoidal transient 21 for just a few oscillations of a single frequency (m/z) component. A representative transient 22 obtained when several different frequencies are combined is shown in FIG. 2B. The m/z value of the ion determines the period (and frequency) of the periodic function. The Single Transient Signal (STS) for single frequency component corresponding to oscillation of ions having mass-to-charge ratio (m/z)1 is approximated by:STS=A sin(2πωt+φ0)  Eq.3where A is a measure of the abundance (quantity) of ions having mass-to-charge ratio (m/z)1 in the trap, ω is the frequency, t is time and φ0 is the initial phase (at t=0). This equation is only an approximation because it does not account for decay of the amplitude and loss of coherence over time.
The information processor 14 performs a Fourier transformation on the received transient. The mathematical method of discrete Fourier transformation may be employed to convert the transient in the time domain (e.g., curve 22 in FIG. 2B), which comprises the mixture of periodic transient signals which result from the mixture of m/z present among the measured ions, into a spectrum in the frequency domain. If desired, at this stage or later, the frequency domain spectrum can be converted into the m/z domain by straightforward calculation. The discrete Fourier transformation produces a spectrum which has a profile point for each frequency or m/z value, and these profile points form a peak at those frequency or m/z positions where an ion signal is detected (i.e. where an ion of corresponding m/z is present in the analyzer).
Mathematically, the Fourier transform outputs a complex number for each profile point (frequency). The complex number comprises a magnitude and a phase angle (often simply termed phase). Alternatively, the complex number at each frequency point may be described as comprising a real component, Re, and an imaginary component, Im. Together, the set of real components, Re, and imaginary components, Im, compose a so-called complex spectrum. It is generally the case that the real component and imaginary component are asymmetrical because the initial phase of the signal at the start of the transient is not zero. Because asymmetrical peaks lead to undesirable low spectral resolution, conventional Fourier transform processing of mass spectral transients has made use of the so-called magnitude spectrum rather than a spectrum based on the real or imaginary components alone. Therefore, in conventional Fourier transform processing of the electrostatic trap transient signal, the phase angle information has often been ignored. To improve the resolution of mass spectra, U.S. Pat. No. 8,853,620 in the name of inventor Lange teaches the generation of enhanced mass spectra that are calculated, after the Fourier-transform generation of real and imaginary complex spectral components, through the combination of a so-called “positive spectrum” (which, in many cases, may be any of a Power spectrum, a Magnitude spectrum or estimates thereof) together with an “absorption spectrum”, which is the real or imaginary component of the complex spectrum after application of an appropriate phase correction that causes the corrected phase to be zero at a peak center.
Regardless of the level of sophistication of the mathematical processing that is employed to convert measured transient signals into mass spectra, the mass resolving power of an electrostatic orbital trapping mass analyzer of the type illustrated in FIGS. 1A-1B or any other electrostatic trapping mass analyzer may be inhibited by accumulation of space charge within the trap. Like any ion trap mass analyzer, there is a finite amount of charge that may be injected into an electrostatic orbital trapping mass analyzer of the type illustrated in FIGS. 1A-1B while still attaining a given level of performance. In a very general sense, the buildup of charge density within a trap produces perturbations of the electric field within the measurement cavity 17 that causes local deviations of the form of the field from the theoretical form given by Eq. 1. More specifically, interactions between ions that are caused by increase in the density of space charge may lead to ion-to-ion transfers of both momentum and energy between ion species of differing m/z ratios. A transfer of momentum may cause disruption of the z-axis oscillatory phase coherence among ions of the same ink value thereby leading to broadened and weakened transient signals, coalescence of mass spectral peaks and consequent loss of spectral resolution. A transfer of energy may cause some ions to prematurely collide with one or the other of the electrodes, thereby contributing to a loss of signal.
The geometric configuration of electrodes within the electrostatic trap mass analyzer illustrated in FIGS. 1A, 1B is more favorable to dispersal of space charge than is three-dimensional radio frequency (RF) quadrupole ion trap. This is because, in the mass analyzer shown in FIGS. 1A, 1B, ions of each ink value are partially angularly dispersed, in the form of an arc, around the spindle electrode 6 within the measurement cavity 17 instead of being confined to a localized central volume (as in a multipole ion trap). Nonetheless, the space charge dispersal parallel to the z-axis is limited, because the z-axis oscillatory amplitude of all ink species is approximately the same, as schematically indicated by cylinder 36 in FIG. 3A. This phenomenon can lead to unacceptably high ion density at the z-axis oscillation extrema, where motion parallel to the z-axis reverses direction for all ions. The accumulated ion density at these “turn-around” zones can lead to situations in which ion species with nearly identical ink ratios move synchronously, thereby leading to peak coalescence in the resulting mass spectra and consequent loss of mass spectral resolution. Many advanced analytical applications require both high resolving power and high signal-to-noise ratios. Therefore, the inventors have recognized a need to improve these performance characteristics, inasmuch as they pertain to some electrostatic traps, by utilizing the available electrostatic trapping volume in a manner that reduces localized accumulation of ion density within the trapping volume. The present invention addresses these needs.